Creep and strength testing of frozen saline fine-grained soils

Abstract

Little creep data is available for frozen, fine-grained saline soils. Salinity accelerates the creep rate of a frozen soil under load and reduces its shear strength owing to the higher unfrozen water contents in the pore phase. Creep data for saline fine-grained soils are required by engineers and scientists working in areas of coastal or offshore permafrost. The data are also used in projects involving artificial freezing of saline soils.This paper describes the results of 34 creep tests and 11 time-dependent strength tests carried out on saline soils. The tests were carried out for two different research programs; consequently, two different fine-grained soil types were used. Most of the tests were carried out in a constant stress and constant temperature creep test environment. Some were completed in a triaxial (constant strain rate) test format, in order to extend the data base to an area of higher strain rates. The range of temperatures was −2.3 to −25 °C, the range of stresses in the creep tests was 30–400 kPa, and the salinity was varied from 0–35 parts per thousand (ppt). Particular attention was paid to the lower stresses (30–200 kPa) and intermediate to high salinities (18–35 ppt). A high percentage of the creep tests was completed in the temperature range −5 to −10 °C, as this appears to be a typical ground temperature in coastal permafrost areas in Arctic Canada.Results indicate that the presence of pore fluid with a salinity approaching that of seawater causes a 10- to 100-fold increase in uniaxial creep rate, which can result in significant decreases in shear strength and foundation bearing capacity in saline permafrost areas.Soil resistivity tests were also carried out on frozen samples of different salinities. A strong correlation between resistivity and pore water salinity emerged. This has application in the interpretation of results from electrical resistivity surveys in permafrost. Key words: permafrost, saline, creep, strength, resistivity, fine-grained, offshore permafrost.